Magnetic disks made of aluminum alloys which are used for the storage units of computers are produced from an aluminum alloy substrate obtained using the JIS 5086 alloy (made of Mg: 3.5 to 4.5 mass %, Fe≦0.50 mass %, Si≦0.40 mass %, Mn: 0.20 to 0.70 mass %, Cr: 0.05 to 0.25 mass %, Cu≦0.10 mass %, Ti≦0.15 mass %, Zn≦0.25 mass % with a balance being Al and inevitable impurities, which has excellent mechanical properties and excellent formability as well as excellent plating property, an aluminum alloy substrate in which the amounts of Fe, Si and the like, which are impurities of the JIS 5086 alloy, are limited to make the intermetallic compounds in the matrix small, or an aluminum alloy substrate to which Cu or Zn has been added to improve the plating property and the like.
A general magnetic disk made of an aluminum alloy is produced by first producing a circular aluminum alloy substrate, plating the circular aluminum alloy substrate and then sputtering a magnetic material to the substrate surface.
For example, a magnetic disk made of the JIS5086 aluminum alloy is produced by the following steps. First, the aluminum alloy is cast, and the cast slab is hot rolled and then cold rolled. The alloy is annealed according to the need, and a rolled material is thus produced. Next, a circular shape is punched out of the rolled material, and the circular aluminum alloy plates are piled. The pile is subjected to pressure annealing where the pile is annealed while pressure is applied to the pile from top and bottom to flatten the pile, and a circular aluminum alloy substrate is thus produced.
Thus produced circular aluminum alloy substrate is subjected to lathing, grinding, degreasing, etching and zincating (Zn substitution treatment), which are pre-treatment, and then to base treatment, namely electroless plating with Ni—P, which are hard nonmagnetic metals. The plated surface is polished, and then a magnetic material is sputtered. Thus, a magnetic disk made of an aluminum alloy is produced.
Recently, it has been desired that magnetic disks have a larger capacity and a higher density due to the needs for multimedia and the like, and an areal memory density of 2 Tb/in2 will be achieved in the near future. In order to increase the memory density of a magnetic disk, it is necessary to reduce the number of plating pits (holes) on the magnetic disk surface, which cause errors when the data are read, and it is desired that the plated surface be highly flat and smooth.
Large hollows in the surface of an aluminum alloy substrate are known to be a cause of the generation of plating pits, and it has been found that the large hollows are created when foreign materials on the substrate surface, such as coarse non-metal inclusions and intermetallic compounds, fall off during grinding or pre-plating treatment.
Due to the circumstances, the reduction in number of the foreign materials in the aluminum alloy substrate has been greatly desired recently, and investigations on this point have been made. PTL 1 describes a method for micronizing the foreign materials such as an Al—Fe—Mn-based intermetallic compound by increasing the cooling rate during the solidification in the casting step.
PTL 2 describes a method for reducing the amounts of Ti, V and Zr, which are impurities contained in molten aluminum, prior to purification by segregation by effectively utilizing the reaction with B. By using raw metal with high purity produced by the method as a starting material for an aluminum alloy plate for a magnetic disk substrate, the generation of foreign materials such as TiB2, VB2 and ZrB2 is inhibited.